I've been using Photoshop for over 15 years and am I guess what you would call an expert. However, I do have an embarrassing secret--I never took the time to learn Illustrator! There have been some days where I wish I knew how to use it, but I have been able to get by with Photoshop.
Honestly, should I take the time to learn Illustrator now or do you all think that Photoshop is really all that is needed? With Photoshop's SMART OBJECT feature (Smart Objects preserve an image’s source content with all its original characteristics, enabling you to perform nondestructive editing to the layer), vector text and shapes, robust pen tool, and today's high resolution computer monitors (now you can truly edit your stuff at 300-600dpi for scalability), is there any good reason to learn Illustrator?
I do, admit, that for logo design it seems Illustrator would be beneficial due to the brushs, but again, one can get by nicely with just Photoshop.
You've been in the game a long time, my guess is that if you have been able to get by until now without learning Illustrator there's probably not a burning need. There may be a desire to challenge yourself or explore but it doesn't sound like there is a big NEED.
Having said that it is always nice to know where the strengths and weakness of any piece of software lie. Illustrator has its role just as Photoshop does. You probably already have many of the skills required to using Illustrator effectively because of your high use of Photoshop. I'd take the time to work through a couple of logo projects even if it is just for fun. Once you understand some of the possibilities you'll know when it is best to switch out to Illustrator or whether you can pull off a job in Photoshop alone. There's so much more overlap than there used to be that the waters are much more 'muddy' over this issue than there used to be.
Personally I was in the same camp as you until I landed a job where I was forced to use Illustrator on a daily basis. I'm still not a heavy user but I can get what I need to done easily. I didn't find it took very long to reach that point given my years of experience in Photoshop.
I would say go for it. Learn it, because you really wouldn't be hurting yourself in doing so. With your experience in Photoshop, Illustrator and the way it works should be a breeze, you will find some extra tricks and fun things you can use, and you can add it to your Repertoire/resume.
I'm not highly proficient at either, but I find Illustrator much easier to use than Photoshop (for me, much more intuitive and understandable). I particularly like the vectors in Illustrator and how robust they are. I don't know how Ps's compare, but I love the nice, flexible scaling vector shapes in Illustrator and always feel restricted when going into Ps--especially with things like layout. I guess if you are tuned in to Ps and how it works, getting used to Ai might be strange for you. For me, it's the other way around.
All the same, as Slackr said, they are getting more similar all the time, and I wouldn't think it would be hard to get going with the basics in minutes.
Illustrator is a much more powerful vector editing tool than photoshop is. I would highly recommend getting the hang of it. I once upon a time only knew how to use photoshop and used it for everything. I would now only use photoshop for what it's name suggests -editing photos. Illustrator is much more adept at creating shapes and simple gradients - and that lossless scaleability makes it great for logo and icon design. For most projects these days I'll use a minimum of both photoshop and illustrator and would not limit myself to using only piece software.
if I can contribute, I'm a newbie here, and have mostly used Corel's suite, rather than Adobe's, but it depends on whether you work with vector graphics (logoes, large prints), or in bitmap graphics (photo editing and manipulation). If you want to make a good logo, do it in Illustrator (or Corel in my case :P), if you want to make your girfriend look like an alien, use Photoshop, right?
If you've been getting by with Photoshop for 15 years, you don't need to learn Illustrator.
Should you? Meh, I've been struggling with this question as well. I know Photoshop (but, I'm sure, not at the level you're at), but not Illustrator. After learning After Effects, I wasn't really looking forward to sitting down and learning another Adobe program, simply because of the hassle.
It's a personal choice.
In my opinion, vector images (maybe because of the SVG standardization) are becoming more and more popular, as are cartoon-esque illustrations.
I think this discussion raises another question. The OP knows Photoshop...does he need to learn Fireworks as well?
The transition or learning curve to Illustrator won't be as harsh as you think it would be having 15 years under your belt. You could probably go to your local learning institution and get a 101 class on Illustrator to learn the basic tricks and you'd be able to figure out the rest.
Though you are expert in Photoshop yet you have learn illustrator. Illustrator is very helpful soft to create a smooth design. For vector design must you have need Illustrator. Thanks.
It's sad that we even have both. Why can't Photoshop just slim down, drop 50+ pointless features and add vector support?